An index whereby excesses or deficiencies of precipitation are determined in relation to average climate values. The index takes in to account precipitation, potential and actual evapotranspiration, infiltration of water into the soil, and runoff.

Low pressure systems that originate in the panhandle region of Texas and Oklahoma which initially move east and then "hook" or recurve more northeast toward the upper Midwest or Great Lakes region. In winter, these systems usually deposit heavy snows north of their surface track. Thunderstorms may be found south of the track.

Pancake Ice

Circular flat pieces of ice with a raised rim; the shape and rim are due to repeated collisions.

Parapet Wall

A solid wall built along the top of the dam for ornament, safety, or to prevent overtopping.

Partial-Duration Flood Series

A list of all flood peaks that exceed a chosen base stage or discharge, regardless of the number of peaks occurring in a year.

The movement of water, under hydrostatic pressure, through the interstices of a rock or soil, except the movement through large openings such as caves. In other words, the movement of water within the soil.

Percolation Deep

In irrigation or farming practice, the amount of water that passes below the root zone of the crop or vegetation.

Percolation Path

The course followed by water moving or percolating through any other permeable material, or under a dam which rests upon a permeable foundation.

Percolation Rate

The rate, usually expressed as a velocity, at which water moves through saturated granular material. The term is also applied to quantity per unit of time of such movement, and has been used erroneously to designate Infiltration Rate or Infiltration Capacity.

Perennial Stream

A stream that flows all year round. Compare intermittent stream.

Perigee

The closest distance between moon and earth or the earth and sun.

Permanent Control

A stream gaging control which is substantially unchanging and is not appreciably affected by scour, fill, or backwater.

Permanent Monument

Fixed monuments placed away from the dam which allow movements in horizontal and vertical control points on the dam to be monitored by using accurate survey procedures.

Permeability

The ability of a material to transmit fluid through its pores when subjected to a difference in head.

Permeability Coefficient

The rate of flow of a fluid through a cross section of a porous mass under a unit hydraulic gradient, at a temperature of 60°F (16°C). The standard coefficient of permeability used in hydrologic work in Meinzer's Units is defined as the rate of flow of water at 60°F (16°C), in gallons per day, through a cross section of 1 ft2, under a hydraulic gradient of 100%. A related coefficient, which may be called the Field Coefficient of Permeability, is defined as the rate of flow of water, in gallons a day, under prevailing conditions, through each foot of thickness of a given aquifer in a width of 1 mile, for each foot per mile of hydraulic gradient.

Permeameter

A laboratory instrument for determining permeability by measuring the discharge through a sample of the material when a known hydraulic head is applied.

Persistence

The length of time during which a signal is visible on a radar display.

Pervious Zone

A part of the cross section of an embankment dam comprising material of high permeability.

Phase

A particular angular stage or point of advancement in a cycle; the fractional part of the angular period through which the wave has advanced, measured from the phase reference.

Phase Shift

The angular difference of two periodic functions.

Phengophobia

The fear of daylight or sunshine.

Phiezometer

An instrument used to measure pressure head in a conduit, tank, soil, etc. They are used in dams to measure the level of saturation.

Phreatic Surface

The free surface of ground water at atmospheric pressure.

Phreatic Water

Water within the earth that supplies wells and springs; water in the zone of saturation where all openings in rocks and soil are filled, the upper surface of which forms the water table. Also termed Groundwater.

Phreatic Zone

The locus of points below the water table where soil pores are filled with water. This is also called the zone of saturation.

Phreatophyte

A plant that habitually obtains its water supply from the zone of saturation, either directly or through the capillary fringe.

Pibal

An acronym for pilot-balloon observation. A method of winds aloft observation; that is, the determination of wind speeds and directions in the atmosphere above a station. This is done by reading the elevation and azimuth angles of a theodolite while visually tracking a pilot balloon.

Piezometer

An instrument for measuring pressure head in a conduit, tank, soil, etc. It usually consists of a small pipe or tube tapped into the side of the container, the inside end being flush with, and normal to, the water face of the container, connected with a manometer pressure gage, mercury of water column, or other device for indicating pressure head.

Confined groundwater is usually under pressure because of the weight of the overburden and the hydrostatic head. If a well penetrates the confining layer, water will rise to this level, the piezometric level, the artesian equivalent of the water table. If the piezometric level is above ground level, the well discharges as a flowing well, artesian well, or a spring.

A small balloon whose ascent at a constant rate is followed by a theodolite in order to obtain data for the computation of the speed and direction of winds at various levels in the upper air above the station.

A device for measuring the velocity of flowing water using the velocity head of the stream as an index of velocity. It consists essentially of an orifice held to a point upstream in the water, connected with a tube in which the rise of water due to velocity head may be observed and measured. It also may be constructed with an upstream and downstream orifice, with two water columns, in which case the difference in height of water column in the tubes is the index of velocity.

Plow Wind

A term used in the midwestern United States to describe strong, straight-line winds associated with the downdrafts spreading out in advance of squall lines and thunderstorms. Resulting damage is usually confined to narrow zones like that caused by tornadoes; however, the winds are all in one direction (straight-line winds).

Pluvial

In hydrology, anything that is brought about directly by precipitation.

Pluviophobia

The fear of rain or of being rained on. See Ombrophobia.

Point Discharge

Instantaneous rate of discharge, in contrast to the mean rate for an interval of time.

Point Precipitation

Precipitation at a particular site, in contrast to the mean precipitation over an area.

Polar Front

A semipermanent, semicontinuous front that separates tropical air masses from polar air masses.

Polar Jet Stream

A jet stream that is associated with the polar front in the middle and high latitudes. It is usually located at altitudes between 30,000 and 40,000 feet (9,000 and 12,000 meters).

Polar Orbiting Satellite

A weather satellite which travels over both poles each time it orbits the Earth. It orbits about 530 miles (850 kilometers) above the Earth's surface.

Polar Vortex

A circumpolar wind circulation which isolates the Antarctic continent during the cold Southern Hemisphere winter.

Polarization Radar

A radar which takes advantage of ways in which the transmitted waves' polarization affect the backscattering. Such radars may alternately transmit horizontal and vertically polarized beams, and measure differential reflectivity.

Pondage

The holding back of water for later release for power development above the dam of a hydroelectric plant to equalize daily or weekly fluctuations of streamflow, or to permit irregular hourly use of water by the wheels to care for fluctuations in the load demand.

In general the holding back of water for later releases.

The storage capacity available for the use of such water.

Ponding

In flat areas, runoff collects, or ponds in depression and cannot drain out. Flood waters must infiltrate slowly into the soil, evaporate, or be pumped out.

Pool Height

The height of the water behind a dam. (Various datum may be used and various pool height may be used, e.g., conservation pool, flood control pool, etc.).

Slang for showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating. Individual thunderstorms typically are of the type sometimes referred to as air-mass thunderstorms. These thunderstorms are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.

Pore Pressure

The interstitial pressure of water within a mass of soil, rock, or concrete.

Porosity

The ratio of pore volume to total volume of the formation. Sandy soils have large pores and a higher porosity than clays and other fine-grained soils.

An index of the void characteristics of a soil or stream as pertaining to percolation; degree of previousness.

Positive Area

The area on a sounding representing the layer in which a lifted parcel would be warmer than the environment; thus, the area between the environmental temperature profile and the path of the lifted parcel. Positive area is a measure of the energy available for convection; see CAPE.

Positive Cloud to Ground (CG) Lightning.

A CG flash that delivers positive charge to the ground, as opposed to the more common negative charge. Positive CGs have been found to occur more frequently in some severe thunderstorms. Their occurrence is detectable by most lightning detection networks, but visually it is not considered possible to distinguish between a positive CG and a negative CG.

Positive-Tilt Trough

An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.

Positive Vorticity Advection (PVA)

A region of positive vorticity usually several hundred of kilometers wide on a upper level chart that moves with the general wind flow. It aids in weather prediction by showing where regions of rising air. This usually results in clouds and precipitation.

Post-storm Report

A report issued by a local National Weather Service office summarizing the impact of a tropical cyclone on its forecast area. These reports include information on observed winds, pressures, storm surges, rainfall, tornadoes, damage and casualties.

The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.

Potential Vorticity

This plays an important role in the generation of vorticity in cyclogenesis, especially along the polar front. It is also very useful in tracing intrusions of stratospheric air deep into the troposphere in the vicinity of jet streaks.

PPINE

Plan Position Indicates No Echoes, referring to the fact that a radar detects no precipitation within its range. An intensity-modulated display on which echo signals are shown in plan view with range and azimuth angle displayed in polar coordinates, forming a map-like display. Each PPI is taken at a single, fixed elevation angle, and thus forms a cone of coverage in space. PPIs may be run in sequence, creating a volume scan.

It measures the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column usually extending from the surface to 300 mb.

Precipitation

1. The process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth as rain, sleet, snow, hail, etc.2. As used in hydrology, precipitation is the discharge of water, in a liquid or solid state, out of the atmosphere, generally onto a land or water surface. It is the common process by which atmospheric water becomes surface, or subsurface water. The term "precipitation" is also commonly used to designate the quantity of water that is precipitated. Precipitation includes rainfall, snow, hail, and sleet, and is therefore a more general term than rainfall.

Precipitation Attenuation

The loss of energy that radar beam experiences as it passes through an area of precipitation.

Predominant Wind

It is the wind that in the forecasters judgment generates (or is expected to generate) the local component of the significant sea conditions across the forecast area.

It is often the first serious indication that a hurricane is approaching. It is a generally a straight line and resembles a squall-line that occurs with a mid-latitude cold front. It is as much as 50 miles (80 kilometers) or even more before the first ragged rain echoes of the hurricane's bands and is usually about 100 to 200 miles (160 to 320 kilometers) ahead of the eye, but it has been observed to be as much as 500 miles (800 kilometers) ahead of the eye in monstrous size hurricanes.

Preliminary Notification of Forthcoming Watch (SAW)

This is an alerting message issued by the Storm Prediction Center in Norman, Oklahoma that either a Tornado or Severe Thunderstorm Watch is about to be issued. This product will contain the distances of the axis coordinates in statue miles, the aviation coordinates will be given in nautical miles, the half widths, valid times in Coordinated Universal Time (UTC), hail size (except when it is a Tornado Watch associated with a Hurricane) at the surface and aloft, the surface wind gusts in knots, maximum tops, and the mean wind vector. As the name indicates, this is a preliminary notification not the actual watch itself. Since the affected local National Weather Service Office has the final say on which counties will be included in the watch, it should be only used as a reference product.

Fire applied to wildland fuels, in a definite place for a specific purpose under exacting weather and fuel conditions (the prescription), to achieve a specific objective of resource management.

Present Movement

The best estimate of the movement of the center of a tropical cyclone at a given time and given position. This estimate does not reflect the short-period, small scale oscillations of the cyclone center.

Pressure

The force exerted by the weight of the atmosphere, also known as atmospheric pressure. When measured on a barometer, it is referred to as barometric pressure and it is expressed in inches of mercury, millibars, or kiloPascals.

Pressure Change

The net difference between pressure readings at the beginning and ending of a specified interval of time.

Pressure Couplet

It is an area where you have a high pressure area located adjacent to a low pressure area.

Pressure Falling Rapidly

A decrease in station pressure at a rate of 0.06" (1.5 mm) of mercury or more per hour which totals 0.02" (0.05 mm) or more.

Pressure Gradient

The amount of pressure change occurring over a given distance.

Pressure Gradient Force

A three-dimensional force vector operating in the atmosphere that accelerates air parcels away from regions of high pressure and toward regions of low pressure in response to an air pressure gradient. Usually resolved into vertical and horizontal components.

Pressure Head

Energy contained by fluid because of its pressure, usually expressed in feet of fluid (foot pounds per pound).

Floating sea, river, or lake ice that has been deformed, altered, or forced upward in pressure ridges by the lateral stresses of any combination of wind, water currents, tides, waves, and surf.

Pressure Jump

A sudden, sharp increase in atmospheric pressure, typically occurring along an active front and preceding a storm.

Pressure Ridge

A discernible rise or ridge, up to 90 feet (30 meters) high and sometimes several miles (kilometers) long, in pressure ice.

Pressure Relief Pipes

Pipes used to relieve uplift or pore pressure in a dam foundation or in the dam structure.

Pressure Rising Rapidly

An increase in station pressure at a rate of 0.06" (1.5 mm) of mercury or more per hour which totals 0.02" (0.05 mm) or more.

Pressure Tendency

The character and amount of atmospheric pressure change during a specified period of time, usually 3-hour period preceding an observation.

Pressure Unsteady

A pressure that fluctuates by 0.03" (0.76 mm) of mercury or more from the mean pressure during the period of measurement.

Prevailing Visibility

The visibility that is considered representative of conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.

Prevailing Westerlies

Winds in the middle latitudes (approximately 30° to 60° N/S) that generally blow from west to east.

Prevailing Wind

A wind that consistently blows from one direction more than from any other.

Price Current Meter

A current meter with a series of conical cups fastened to a flat framework through which a pin extends. The pin sets in the framework of the meter, and the cups are rotated around it in a horizontal plane by the flowing water, registering the number of revolutions by acoustical or electrical devices, from which the velocity of the water may be computed.

It is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Measurable precipitation is defined as equal to or greater than 0.01" or 0.25 mm. Normally, the period of time is 12 hours, unless specified otherwise. The forecast area, or zone, is generally considered to be a county. In some geographically unique areas (mountains), the forecast area/zone may consist of a portion of a county or two or more counties.

At times, some NWS forecasters will use "occasional" or "periods of" to describe a precipitation event that has a high probability of occurrence, i.e., they expect any given location in a forecast zone area to most likely have precipitation, but it will be of an on and off nature. Usually, away from the mountains, each and every county is a forecast zone area itself.

Probability of Tropical Cyclone Conditions (SPF)

The probability, in percent, that the cyclone center will pass within 50 miles to the right or 75 miles to the left of the listed location within the indicated time period when looking at the coast in the direction of the cyclone's movement.

Probabilistic Quantitative Precipitation Forecast (PQPF)

A form of QPF (see below) that includes an assigned probability of occurrence for each numerical value in the forecast product.

Product Resolution

The smallest spatial increment or data element that is distinguishable in a given Doppler radar product.

Profiler

An instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.

Prognostic Discussion (PMD)

This Hydrometeorological Prediction Center discussion may include analysis of numerical and statistical models, meteorological circulation patterns and trends, and confidence factors. Reference is usually made to the manually produced 6- to 10-day Northern Hemisphere prognoses for mean 500 millibar heights and mean 500 millibar height anomalies. Discussions may also refer to the method of operational ensemble predictions.

They are characterized by a short curved squall line oriented nearly perpendicular to the mean wind direction with a bulge in the general direction of the mean flow. Downburst activity occurs along the bulging portion of the line. This type of derecho typically occurs in the warm season (May through August) and is most frequent in a zone extending from eastern South Dakota to the upper Ohio Valley. The severe wind storms typically begin during the afternoon and continue into the evening hours. Several hours typically pass between initial convection and the first wind damage report.

Propagation

The transmission of electromagnetic energy as waves through or along a medium.

Property Protection

Measures that are undertaken usually by property owners in order to prevent, or reduce flood damage. Property protection measures are often inexpensive for the community because they are implemented by or cost-shared with property owners. In many cases the buildings' appearance or use is unaffected, so these measurements are particularity appropriate for historical sites and landmarks. These measures include relocation and acquisition, flood proofing, and buying flood insurance.

A boundary between a supercell's inflow region and the forward-flank downdraft (or FFD). It extends outward from at or near the mesocyclone center, usually toward the east or southeast, and normally is either nearly stationary or moves northward or northeastward ahead of the mesocyclone. See pseudo-cold front and beaver tail.

Psychrometer

An instrument used to measure the water vapor content of the air. It consists of two thermometers, one of which is an ordinary glass thermometer, while the other has its bulb covered with a jacket of clean muslin which is saturated with distilled water prior to use. After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.

Public educational information and activities, such as storm safety rules, awareness activities, storm drills, etc, or

Information regarding service changes, service limitations, interruptions due to reduced or lost power or equipment outages, or special information clarifying interpretation of NWS data.

For example, this product may be used to inform users of radar equipment outages or special information clarifying interpretation of radar data originating from an unusual source which may be mistaken for precipitation (such as chaff drops, smoke plumes, etc., that produces echoes on the radar display.

Public Severe Weather Outlook (PWO)

These are issued when the Storm Prediction Center in Norman, Oklahoma anticipates an especially significant and/or widespread outbreak of severe weather. This outlook will stress the seriousness of the situation, defines the threat area, and provides information on the timing of the outbreak. The lead time on this outlook is normally less than 36 hours prior to the severe weather event.

Puddle

1. The act of compacting earth, soil clay, etc., by mixing them with water and rolling or tamping the mixture.2. A compact mass of earth, soil, clay, or a mixture of material, which has been compacted through the addition of water, rolling and tamping. This makes the material less permeable.
3. A small pool of water, usually a few inches in depth and from several inches to several feet in it greatest dimension.

Puget Sound Convergence Zone (PSCZ)

A situation where wind forced around the Olympic Mountains converges over the Puget Sound. Causes extreme variability in weather conditions around Seattle, Washington with some areas of sunshine and others in clouds and rain.

Pulse

A short burst of electromagnetic energy that a radar sends out in a straight line to detect a precipitation target. The straight line that this pulse travels along is called a radar beam.

Pulse Duration

The time in which a radar pulse lasts. The pulse duration can be multiplied by the speed of light to determine the pulse length or pulse width.

The linear distance in range occupied by an individual pulse from a radar. h = c * t , where t is the duration of the transmitted pulse, c is the speed of light, h is the length of the pulse in space. Note, in the radar equation, the length h/2 is actually used for calculating pulse volume because we are only interested in signals that arrive back at the radar simultaneously. This is also called a pulse width.

Pulse-Pair Processing

Nickname for the technique of mean velocity estimation by calculation of the signal complex covariance argument. The calculation requires two consecutive pulses, hence "pulse-pair".

Pulse Radar

A type of radar, designed to facilitate range (distance) measurements, in which are transmitted energy emitted in periodic, brief transmission.

These are single cell thunderstorms which produce brief periods of severe weather (1" hail, wind gusts in excess of 58 miles an hour, or a tornado).

Pulse Storm

A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top and cyclic storm.

Pulse Repetition Time (PRT)

The time elapsed between pulses by the radar. This is also called the pulse interval.